Glass is desirable for numerous properties and applications, including optical clarity and overall visual appearance. In at least some applications, it may be desirable to optimize certain optical properties (such as light transmission, reflection and/or absorption properties). For example, in certain embodiments, a reduction of the reflection of light by the surface of a glass substrate may be desired, such as for windows (e.g., storefront windows), display cases, photovoltaic (PV) devices (e.g., solar cells), pictures frames, and so forth.
Photovoltaic devices, such as solar cells (and modules therefore), are known in the art. Glass is an integral part of most common commercial photovoltaic modules, including both crystalline and thin film types. A solar cell/module may include, for example, a photoelectric transfer film made up of one or more layers located between a pair of substrates. One or more of the substrates may be glass, and the photoelectric transfer film (typically semiconductor) is for converting solar energy to electricity. Example solar cells are disclosed in U.S. Pat. Nos. 4,510,344, 4,806,436, 5,977,477, and 6,506,622, and JP 07-122764, the disclosures of which are hereby incorporated herein by reference.
In a solar cell where the substrate is glass, incoming radiation passes through the incident glass substrate of the solar cell before reaching the active layer(s) (e.g., photoelectric transfer film such as a semiconductor) of the solar cell. Radiation that is reflected by the incident glass substrate does not make its way into the active layer(s) of the solar cell, thereby resulting in a less efficient solar cell. Thus, it would be desirable in at least certain embodiments to decrease the amount of radiation that is reflected by the incident substrate, thereby increasing the amount of radiation that makes its way to the active layer(s) of the solar cell. In particular, the power output of a solar cell or PV module may be dependant upon the amount of light, or number of photons, within a specific range of the solar spectrum that pass through the incident glass substrate and reach the PV semiconductor.
Because the power output of the module may depend upon the amount of solar light that passes through the glass and reaches the PV semiconductor, certain attempts have been made to boost overall solar transmission through the glass used in PV modules. One attempt is the use of iron-free or “clear” glass, which may increase the amount of solar light transmission when compared to regular float glass, through absorption minimization.
In another attempt to address this problem, an AR coating is deposited on a glass substrate, which may be deposited on either side of the glass substrate. An AR coating may increase transmission of light through the light incident substrate, and thus the power of a PV module.
Conventional wet chemical techniques to produce AR oxide coatings may employ sol-gel processes involving hydrolysis and condensation reactions of alkoxides, such as titanium alkoxides and silicon alkoxides. Precursor coatings that may be formed from these sols may be fired at elevated temperatures to convert them to oxide coatings. During thermal processing of coatings, heating profiles of gradual temperature ramp rates may be employed to burn off organic content and form oxide coatings.
Typically in these processes, liquid sols may be aged for several hours after preparation in order to ensure thorough hydrolysis of precursor alkoxides. The stability of sols may be affected by several factors including pH, water content, concentration of solids, etc. Chelating ligands, such as 2,4-pentanedione, may be added in some instances to prolong shelf life of the liquid sols. However, producing stable sols at manufacturing volumes can be challenging. While the shelf life of sols may be influenced by storage and transportation conditions, the useful pot life during processing may be affected by loss of volatiles and exposure to ambient humidity, etc.
In the case of manufacture of multiple-layer coatings, each layer must be heat cured in order to develop sufficient solvent resilience before the next layer is deposited. Thus, the process of making multi-layered precursor coatings using conventional sol-gel processes may become complicated and tedious, and may adversely affect the cost of production. Accordingly, there exists a need for a method to produce multi-layer precursor oxide coatings without the need for traditional thermal processing of each layer prior to depositing subsequent layers.
The inventor has now discovered novel methods to produce multi-layered AR coatings on a substrate, which may increase transmission of light through the substrate.